This invention relates to an oxygen sensor used in detecting the oxygen concentration in a gas such as the exhaust gas from an internal combustion engine of automotive vehicles. More particularly, it relates to a solid electrolyte consisting essentially of sintered zirconia, which has a good stability of crystal structure and a good oxygen ion conductivity.
Among the solid electrolytes of this sort for use in an oxygen sensor, there has been known a sintered body of stabilized zirconia which is prepared by mixing zirconia with a stabilizer such as yttria, calcia, or ytterbia followed by sintering at high temperatures, as disclosed in U.S. Pat. No. 4,219,359 (Miwa et al.).
However, it has been found that the solid electrolyte composed of said stabilized zirconia, although expected to be of a crystal structure constituted by a cubic phase alone, sometimes exhibits a poor oxygen ion conductivity, an insufficient stability of crystal structure leading to destruction of the structure, and, moreover, a very poor thermal shock resistance.
The inventors examined a sintered body of stabilized zirconia, which had presented the above problems, by finely pulverizing said sintered body as a sample and analyzing the pulverized sample by the powder X-ray diffraction method. A monoclinic phase was detected beside the cubic phase. This suggests that while only the cubic phase is present initially, partial phase transition from cubic to monoclinic takes place under the practical use conditions, e.g. when a temperature change in the atmosphere was made or continuous operation in a low temperature range (e.g. about 200.degree. to 300.degree. C.) carried out. The partial transition of a cubic phase into a monoclinic is accompanied with a volume expansion which tends to give rise to the destruction of the structure of the sintered body. The oxygen ion conductivity seems to be reduced due to such a destruction as well as the presence of the monoclinic phase having an intrinsically low oxygen ion conductivity. Thus, some of the sintered zirconia forming a solid electrolyte undergo the partial transition from a cubic phase to monoclinic phase during practical use, even though they initially had a crystal structure of the cubic phase alone. Therefore, this presents an important problem of how to reduce the proportion of the monoclinic phase.
Thus, the inventors have found the fact that when the cubic-phase sintered zirconia was finely ground, there may be detected a monoclinic phase.